1. Field of the Invention
The field of the invention is that of blending dispensers for liquids. More particularly, the field is that of blending dispensers capable of dispensing a variety of different octane fuels.
2. Description of the Related Art
In liquid dispensing devices, for example gasoline fuel pumps, different grades of gasolines are conventionally mixed to provide a specific blend. Using two gasoline sources, one supply tank containing gasoline having a first octane rating, for instance, 95, and the other supply tank containing gasoline having another octane rating, for instance, 83, conventional dispensers produce gasoline blends having a desired octane rating in the range of 83 to 95 by measuring the volume of each octane being dispensed and adjusting accordingly.
The following example uses 83 and 95 octane gasolines as the component fuels because they represent a common prior art range of octanes; however, a variety of ranges of octanes have been used to mix and provide blends of gasoline. For example, gasolines of various octane ratings can be provided by mixing component gasolines in the following percentages:
______________________________________ Octane % of 83 Octane % of 95 Octane ______________________________________ 83 100 0 87 67 33 89 50 50 91 33 67 95 0 100 ______________________________________
In prior art systems, such as U.S. Pat. No. 4,876,653, issued to McSpadden, a proportional control system has been used to check the blending ratio periodically by observing the instantaneous flow rates of the two component streams of fluid (e.g. from the 83 and 95 octane tanks). An electronic controller observes the instantaneous flow rates by receiving signals from flow meters in the two flow lines. When the instantaneous flow rates substantially deviate from those needed to provide the desired mixture, the valves which control the component streams are adjusted accordingly. This results in a dispenser which tends to deliver an instantaneous blend ratio equal to the desired mixture only after a significant portion of the desired quantity of fuel has been dispensed, assuming that liquid pressure remains relatively constant. Thus a significant quantity of the dispensed fuel may have an incorrect blend or mixture. However, such proportional control systems only correct for errors in the instantaneous flow rate and do not account for errors in the fuel which has already been dispensed so that the total amount of dispensed fuel will be a desired blend. Moreover, such systems are dependent on having components stocks which are of the correct octane. If the stock fluids are not of the correct octane, which is often the case, the resulting mix will be correspondingly incorrect.
More recent prior art, such as U.S. Pat. Nos. 5,038,971 and 5,125,533, both assigned to the present assignee, greatly improve on this concept by measuring the flow rates of two component streams and using a proportional-integral-differential (PID) control algorithm. The PID control algorithm lessens the amount of deviation from the desired ratio, and results in a blend closer to the actual desired octane. However, such a system still assumes the component fuels possess their claimed octane, and the octanes of each component fuel are not actually measured but rather the volume of each component fuel is adjusted if the flow rates diverge past certain parameters.